The present invention relates to multilayer printed circuit boards, and more specifically to a component suitable for use in manufacturing multilayer printed circuit boards, and to a method used for its manufacture.
Multilayer printed circuit boards are widely used in modern electronic equipment, particularly that which is used professionally, since, amongst other things, they allow the assembly of electronic components with a greater density, thus reducing the bulk of the equipment itself. The conventional method for manufacturing multilayer printed circuit boards is well known to those acquainted with the sector art and for this reason a detailed description will not be given here, for a better understanding of the present invention, it may be useful to recall that in the step of preparing the so-called xe2x80x9cstacksxe2x80x9d, a mirror-polished stainless steel plate is used in the hot pressing step, placed as a separator between two separate multilayer circuit boards to which it is fixed by means of a separation film. This is in order to protect the external surfaces of the circuit boards. It also minimises the surface irregularities and makes the temperature distribution uniform.
The separation film is used to prevent any infiltration of the adhesive used between the layers which make up the multilayer circuit boards, and has to be removed at the end of the process, while the mirror-polished stainless steel plate is polished thoroughly before being used again; this procedure is complex and costly, comprising the quality and the efficiency of the manufacturing process.
The U.S. Pat. No. 5,153,050 proposes and describes a variation of the conventional process referred to above, in which provision is made for the use of a specific component consisting of a laminate made typically from two external copper sheets and one internal aluminium sheet, sealed at the edges. At the end of the process, the two copper sheets will each become the outermost conductive layer of two distinct multilayer circuit boards, while the aluminium sheet is a disposable element, which is eliminated at the end of the process itself.
The faces of the two copper sheets and the face of the aluminium sheet placed in reciprocal contact must not be contaminated, i.e. they must be free from all particles (e.g., resin and glass fibre dusts) and adhesive. A light strip of temporary adhesive along the perimeter fixes the non-contaminated faces to each other, thus defining two non-contaminated central zones, protected during the subsequent steps of the work. The aluminium sheet also contributes by giving the whole rigidity, facilitating handling operations.
The process for manufacturing multilayer printed circuit boards using this component is generally designated with the initials C-A-C, meaning Copper-Aluminium-Copper, or with the initials R-A-R in Italian (Rame-Alluminio-Rame).
One inconvenient aspect of the C-A-C process is the high cost of the adhesive, since very specific properties are required, as it has to remain stable and effective during the various steps of the work and it must be easily removable, without leaving any residue at the end of the process. For this reason the manufacturers of multilayer printed circuit boards felt it was necessary to find a cheaper but highly reliable alternative.
Furthermore, in the manufacture of multilayer printed circuit boards, the technology of microdrilling using a xe2x80x9claserxe2x80x9d light ray has recently been developed, also called laser microdrilling for brevity, where by this term is meant the non pass-through drilling of holes with a very small diameter, typically ranging between 0.05 mm and 0.25 mm. These non-through holes, as those acquainted with the sector art will know, are normally made on the outer surfaces of the multilayer printed circuits, and are intended to be suitably metalized in such a way as to electrically connect the circuits made on the outer conductive layer of copper to the circuits made on the inner conductive layers. As laser microdrilling is much more effective when the material to be drilled does not contain glass fiber, the solution most widely adopted recently is to use a component made from a copper sheet covered on one side by a layer of epoxy resin of variable thickness, where this component is known as RCC foil, the abbreviation RCC standing for xe2x80x9cResin Coated Copperxe2x80x9d. In fact, if this component is applied to the outer surfaces of the multilayer printed circuit board in place of the normal copper sheet, drilling with a laser ray is made possible with a much greater efficiency, since under the copper sheet the laser ray does not meet the glass fiber typically present in the insulating layers of the multilayer printed circuit board, but instead meets the resin.
On the other hand, the presence of the layer of resin makes this component particularly delicate to handle, to the extent that simply bending it may lead to microcracks in the layer of resin with resultant product defects.
Use of these RCC components is therefore considerably critical, as their fragility is their worst defect.
The object the present invention is to define a component for the C-A-C multilayer printed circuit board manufacturing process, which is economical, can be made using know, tried and tested systems, having a high level of productivity, and which make if possible to achieve excellent quality and reliability. The component of the invention is obtained by joining the two external copper sheets to the internal aluminium sheet by means of a continuous electro-mechanical or mechanical process, which does not require additional materials (such as adhesive) and guarantees a consistently high level of quality.
This process will preferably consist of ultrasonic welding, subject to appropriate regulation of the power and pressure, or of a knurling operation, subject to the definition of the properties of the imprint.
Another object of the invention is that of defining a method for manufacturing C-A-C type multilayer printed circuit boards based on the use of a component obtained by the joining of the two external copper sheets to the internal aluminium sheet by means of a continuous electro-mechanical or mechanical process, such as ultrasonic welding or knurling.
A further object of the invention is that of defining a multilayer printed circuit board, manufactured by means of a C-A-C type method based on the use of a component made by the joining of the two external copper sheets to the internal aluminium sheet by means of a continuous electro-mechanical or mechanical process, such as ultrasonic welding or knurling. Another object of this invention is to define a component suitable for being used in a process for manufacturing multilayer printed circuits, based on use of the laser microdrilling technology, wherein the said component is capable of overcoming the drawbacks, particularly the fragility, typical of the components used at the moment. The component of the invention is obtained by joining at last one copper sheet with an aluminium sheet by means of a continuous electromechanical or mechanical process, that does not need any additional materials (such as adhesive) and guarantees a consistently high level of quality, and further by depositing a layer of resin on the outer surface of the copper sheet not in contact with the aluminium sheet.
The above mentioned objects are obtained by means of a component, and a manufacturing method characterized as defined in the main claims.
These and other objects, characteristics and advantages of the invention will become evident on the basis of the following description of a preferred embodiment, provided purely by way of an illustrative, non-restrictive example, and with reference to the accompanying drawings.